Subterranean coal beds often contain large quantities of methane. The presence of methane in these subterranean coal deposits presents a safety hazard in coal mining operations, but also presents an opportunity for recovery of a valuable fuel. In the past, coalbed methane was often vented to the atmosphere or flared to reduce the safety risk in mining. More recently, in order to minimize air pollution and maximize economic return from coal bed operations, there has been an increasing focus on recovering methane rather than venting or flaring it. The recovery of coalbed methane is typically accomplished by drilling and completing a gas well into the coal seam and fracturing the well within the coal formation to enhance methane recovery.
Hydraulic fracturing methods for oil and gas wells drilled in a hard rock formation involve injecting a fracturing fluid (e.g., an aqueous gel or an aqueous foam) through the wellbore and against the face of the subterranean formation at pumping rates and pressures sufficient to create or extend cracks in the formation. Typically a proppant (e.g., sand or bauxite) is mixed with the fracture fluid and is carried by the fluid into the fractures. When the pumping rate and pressure are reduced, the fractured formation settles back onto the emplaced proppant, and the proppant holds the fractures open sufficiently to establish a permeable fluid communication channel from the tip of the pack of proppant back to the wellbore.
Fracture stimulation of coalbed methane reservoirs requires techniques quite different from those used in conventional hard-rock reservoirs. The methane in a coal seam is adsorbed to the surface of the coal. At a certain pressure, governed by the Langmuir desorption isotherm, the methane will begin to desorb from the coal. In addition, coal seams are often completely saturated with water. In these cases, large quantities of water must be removed in order to lower the reservoir pressure to a point below the methane desorption pressure. Therefore, a hydraulic fracturing treatment in a coal seam must be designed to produce water effectively.
Maintaining the coal in an oil-wet state facilitates water production. This is because coal is soft and friable. Wells are generally produced at maximum pressure drawdown to reduce the reservoir pressure as quickly as possible. The proppant particles (usually sand) become embedded into the fracture faces due to the increase in closure stress created by the high drawdown pressure. Proppant embedment causes a large quantity of coal fines to be produced. If these fines are water-wet, then they will be easily transported in the water phase during dewatering of the coal bed. The fines will then migrate into the fracture, eventually causing severe reduction of the fracture conductivity. It is therefore important to maintain the coal fines in an oil-wet state, so they will tend to clump together in the presence of water, thereby greatly reducing their mobility. This concept is also critical in the natural fracture (cleat) system of the coal adjacent to the hydraulic fracture. Coal fines will be generated due to shrinkage of the coal, oxidation, etc. These fines can cause plugging of the cleat system, which severely reduces the well productivity and ultimate gas production.
Additives exist that can provide good oil wetting of coal. For example, superior oil wetting in the presence of water can be achieved by methods and materials described in U.S. Pat. No. 5,229,017 (Nimerick and Hinkel). One such commercially available surfactant, denoted surfactant A herein, (available from Schlumberger), comprises a branched tridecyl alcohol with seven moles ethylene oxide (EO) and two moles butylene oxide (BO).
Foamed fracturing fluids are often preferred over non-foamed fracturing fluids in coal seam reservoirs in order to minimize the damage associated with the natural polymers typically present in the base fluid. Nitrogen is most often used as the gaseous phase in the foam fracturing treatments. However, materials that act as good oil-wetters for coal have been proven ineffective in providing stable aqueous foams. For example, surfactant A acts as an anti-foaming agent.
There is a need for improved fracturing fluids and methods that are suitable for use in coal beds to stimulate production of methane.